Method and apparatus for timing correction in communications systems

ABSTRACT

In a wireless communications system, transceivers transmit short bursts to a base station, which determines timing corrections from the time of receipt of the burst and transmits the timing corrections to the respective transceivers. In one aspect, the base station indicates to the transceivers a plurality of time slots, each transceiver selects one of the time slots at random, formats a burst including an indicator of the selected time slot and transmits the burst in that slot. In another aspect, the base station transmits to each transceiver a timing uncertainty value, which determines how the timing correction will be modified by the tranceiver as the interval since last receiving a timing correction increases. Data bursts are transmitted in a format comprising a first unique word, a content field and a second unique word, in that order. The bursts are transmitted in a TDMA channel format which can accommodate both short and long bursts in a block format of constant periodicity.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of application Ser. No. 09/439,348,filed on Nov. 15, 1999, which is incorporated herein by reference in itsentirety.

The present invention relates to communications apparatus and methods,particularly but not exclusively for wireless communications,particularly but not exclusively via satellite.

A problem associated with communications systems in which differenttransmitters share a time-divided channel resource is that timingmisalignments may cause interference between the transmitters. Themisalignments may be caused by drift in the transmitter clocks, orvariations in the propagation delay from different transmitters to acommon receiver. In time-divided multiple access (TDMA) channels, aguard band is usually provided between adjacent time slots, so thattiming differences of less than the guard band time betweentransmissions in adjacent slots do not cause interference. However, theguard bands occupy bandwidth which could otherwise be used to carrytraffic, so reliance on guard bands alone to avoid interference is notsuitable for systems where a high bandwidth channel is shared by manytransmitters.

The document U.S. Pat. No. 5,790,939 describes a TDMA based satellitecommunication system including a timing correction protocol. The systembroadcasts timing corrections lo mobile terminals. Residual errors inthe timing of individual terminals are corrected following transmissionby the mobile terminals in a contention access channel. A gatewaymeasures the residual timing error and reports the error back to therelevant mobile terminal.

Another problem associated with bursts in TDMA channels is that, ifadjacent bursts do overlap, the interference between the burstsgenerally prevents either from being demodulated and decodedsuccessfully. Burst formats for each time slot may include a unique wordwhich aids acquisition of the burst, as described for example in U.S.Pat. No. 5,661,764, but the advantage of the unique word is lost if theburst interferes with an adjacent burst.

The document GB 2270815 describes a cellular mobile radio system with apacket reservation multiple access protocol, in which user traffic canbe carried by both single and double slots, allocated dynamically by thebase station according to load requirements. However, if the slotallocation is entirely flexible and can include slots of differinglengths, the timing alignment of bursts in those slots becomes complex.

According to aspects of the present invention, there is provided atiming correction method in a communications system, in whichtransceivers transmit short bursts to a base station, which determinestiming corrections from the time of receipt of the bursts and transmitsthe timing corrections to the respective transceivers.

In one aspect, the base station indicates to the transceivers aplurality of time slots; each transceiver selects one of the time slotsat random, formats a burst including a indicator of the selected timeslot and transmits the burst In that slot. The base station cantherefore determine the timing with which each transceiver transmittedthe burst, but the probability of collision between bursts is reducedsince they ate spread across the plurality of time slots.

In another aspect, the base station transmits to each transceiver atiming uncertainty value, which determines how the timing correctionwill be modified by the transceiver as the interval since last receivinga timing correction increases. Preferably, if the modificationdetermined by the timing uncertainty value increases beyond apredetermined threshold, the transceiver inhibits transmission otherthan to request a timing correction. These measures advantageouslyreduce the likelihood of interference between slots, due to timingmisalignment.

The above aspects of the present invention extend individually to thoseparts of the method which are carried out by the transceiver, thoseparts which are carried out by the network with which the transmittercommunicates, and apparatus arranged to carry out those individual partsof the method.

According to another aspect of the present invention, there is provideda signal having a format comprising a first unique word, a content fieldand a second unique word, in that order. Preferably, there are no otherfields in the burst before the first or after the second unique wordwhich are necessary for the demodulation and decoding of the burst; thishas the advantage that, if either the beginning or the end of the burstoverlaps with another burst, it may still be possible to read the datacontent of the burst correctly. The content field may carry user dataand/or signalling information. There may be an additional field beforethe first unique word and/or after the second unique word, but these arepreferably auxiliary fields which are not essential to the decoding ofthe content field. For example, there may be a constant power preambleat the beginning of the burst to assist with power control in thetransmitter. This aspect of the present invention extends to methods offormatting and/or transmitting such a signal, and apparatus arranged toperform such methods.

According to another aspect of the present invention, there is provideda TDMA channel format which can accommodate both short and long burstsin a block format of constant periodicity.

Specific embodiments of the present invention will now be described withreference to the accompanying drawings, in which:

FIG. 1 is a diagram of components of a satellite communication systemincorporating embodiments of the present invention;

FIG. 2 shows the channels used for communication between the SAN and theMAN's in a packet data service implemented in the system of FIG. 1;

FIG. 3 is a diagram of transmitter and receiver channel units in asatellite access node (SAN) of the system of FIG. 1;

FIG. 4 is a diagram of transmitter and receiver channel units in aMobile Access Node (MAN) of the system of FIG. 1;

FIGS. 5 a to 5 d show the structure of one of the LESP channels of FIG.4;

FIG. 6 a shows the burst structure of a 5 ms burst in one of the MESPchannels of FIG. 4;

FIG. 6 b shows the burst structure of a 20 ms burst in one of the MESPchannels of FIG. 4;

FIG. 7 is a timing diagram illustrating the operation of an initialtiming correction protocol for correcting the timing of transmissions inthe MESP channels;

FIG. 8 a is a timing diagram illustrating the timing of a transmissionin one of the MESP channels immediately following a timing correction;

FIG. 8 b is a timing diagram illustrating the timing of a transmissionin one of the MESP channels at an interval after a timing correction,where there is timing uncertainty;

FIGS. 9 a to 9 c are timing diagrams showing different collisionscenarios between bursts of a conventional format in adjacent TDMAslots; and

FIGS. 10 a to 10 c are timing diagrams showing the equivalent collisionscenarios between bursts of a format according to an embodiment of thepresent invention.

SYSTEM OVERVIEW

FIG. 1 shows the principal elements of a satellite communications systemin an embodiment of the present invention. A plurality of Mobile AccessNodes (MAN) 2 communicate via a satellite 4 with a satellite earthstation, hereinafter referred to as a Satellite Access Node (SAN) 6. Thesatellite 4 may for example be an Inmarsat-3™ satellite, as describedfor example in the article ‘Launch of a New Generation’ by J R Asker,TRANSAT, Issue 36, January 1996, pages 15 to 18, published by Inmarsat,the contents of which are included herein by reference. The satellite 4is geostationary and projects a plurality of spot beams SB (five spotbeams in the case of an Inmarsat-3™ satellite) and a global beam GB,which encompasses the coverage areas of the spot beams SB, on theearth's surface. The MAN's 2 may be portable satellite terminals havingmanually steerable antennas, of the type currently available for usewith the Inmarsat mini-M™ service but with modifications as describedhereafter. There may be a plurality of SAN's 6 within the coverage areaof each satellite 4 and capable of supporting communications with theMAN's 2 and there may also be further geostationary satellites 4 withcoverage areas which may or may not overlap that of the exemplarysatellite 4. Each SAN 6 may form part of an Inmarsat Land Earth Station(LES) and share RF antennas and modulation/demodulation equipment withconventional parts of the LES. Each SAN 6 provides an interface betweenthe communications link through the satellite 4 and one or moreterrestrial networks 8, so as to connect the MAN's 2 to terrestrialaccess nodes (TAN) 10, which are connectable directly or indirectlythrough further networks to any of a number of communications services,such as Internet, PSTN or ISDN-based services.

Channel Types

FIG. 2 shows the channels used for communication between a sample one ofthe MAN's 2 and the SAN 6. All communications under this packet dataservice from the MAN 2 to the SAN 6 are carried on one or more slots ofone or more TDMA channels, referred to as MESP channels (mobile earthstation—packet channels). Each MESP channel is divided into 40 msblocks, divisible into 20 ms blocks. Each 20 ms block carries either one20 ms burst or four 5 ms bursts, in a format which will be describedbelow.

All communications under this packet data service from the SAN 6 to theMAN 2 are carried on one or more slots of one or more TDM channels,referred to as LESP channels (land earth station—packet channels). Theslots are each 80 ms long, and comprise two subframes of equal length.

For the purposes of channel set-up and other network signalling, the MAN2 also communicates with a network co-ordination station (NCS) 5, as isknown in the Inmarsat Mini-M™ service. The SAN 6 communicates throughthe network 8 to a regional land earth station (RLES) 9 whichcommunicates with the NCS 5 so as to perform channel set-up and othernetwork signalling.

Satellite Link Interface

The present embodiments concern in particular a set of protocols andalgorithms for the interface over the satellite link between the MAN's 2and the SAN 6 to which the MAN's 2 are connected. This interface can beconsidered as a series of communications layers: a physical layer, amedium access control (MAC) layer and a service connection layer.

SAN Channel Unit

FIG. 3 shows the functions within the SAN 6 of a transmitter channelunit ST which performs the transmission of data packets over a singlefrequency channel of the satellite link, and a receiver channel unit SR,which performs the reception of data packets over a single frequencychannel of the satellite link. Preferably, the SAN 6 includes multipletransmitter channel units ST and receiver channel units SR so as to beable to provide communications services to a sufficient number of MAN's2.

A hardware adaptation layer (HAL) 10 provides an interface between thechannel units and higher level software, and controls the settings ofthe channel units. In the transmitter channel unit ST, the HAL 10outputs data bursts Td which are scrambled by a scrambler 12, the outputtiming of which is controlled by a frame timing function 14 which alsoprovides frame timing control signals to the other transmitter channelunits ST. The scrambled data bursts are then redundancy encoded by anencoder 16, by means for example of a turbo encoding algorithm asdescribed in PCT/GB97/03551.

The data and parity bits are output from the encoder 16 to a transmitsynchronising function 18 which outputs the data and parity bits as setsof four bits for modulation by a 16QAM modulator 20. Unique word (UW)symbols are also input to the modulator 20 according to a slot formatwhich is described below. The output timing of the encoder 16, transmitsynchroniser 18 and modulator 20 is controlled by the HAL 10, which alsoselects the frequency of the transmit channel by controlling a transmitfrequency synthesiser 22 to output an upconversion frequency signal.This frequency signal is combined with the output of the modulator 20 atan upconverter 24, the output of which is transmitted by an RF antenna(not shown) to the satellite 4.

In the receiver channel unit SR, a frequency channel is received by anRF antenna (not shown) and downconverted by mixing with a downconversionfrequency signal at a downconverter 26. The downconversion frequencysignal is generated by a reception frequency signal synthesiser 28, theoutput frequency of which is controlled by the HAL 10.

In order to demodulate the received bursts correctly, the timing ofreception of the bursts is predicted by a receive timing controller 29,which receives the frame timing control information from the frametiming function 14 and parameters of the satellite 4 from the HAL 10.These parameters define the position of the satellite 4 and of its beamsand allow the timing of arrival of data bursts from the MAN's 2 to theSAN 6 to be predicted. The propagation delay from the SAN 6 to thesatellite 4 varies cyclically over a 24 hour period as a result of theinclination of the satellite's orbit. This delay variation is similarfor all of the MAN's 2 and is therefore used to modify the referencetiming of the MESP channels, so that the timing of the individual MAN's2 does not need to be modified to compensate for variations in satelliteposition.

The predicted timing information is output to each of the receivechannel units SR. The received bursts are of either 5 ms or 20 msduration according to a scheme controlled by the SAN 6. The HAL 10provides information about the expected slot types to a slot controller32, which also receives information from the receive timing controller29.

FIG. 3 shows separate reception paths for 5 ms and 20 ms bursts;references to functions on each of these paths will be denoted by thesuffixes a and b respectively. The slot controller 32 selects whichreception path to use for each received burst according to the predictedlength of the burst. The burst is demodulated by a 16QAM demodulator 34a/34 b and the timing of the burst is acquired by a UW acquisition stage36 a/36 b. Once the start and end of the burst is determined, the burstis turbo-decoded by a decoder 38 a/38 b and descrambled by a descrambler40 a/40 b. The recovered 5 or 20 ms data burst is then received by theHAL 10.

MAN Channel Unit

FIG. 4 shows the functions within one of the MAN's 2 of a receiverchannel unit MR and a transmitter channel unit MT. The MAN 2 may haveonly one each of the receiver and transmitter channel unit, for reasonsof compactness and cost, but if increased bandwidth capacity isrequired, multiple receiver and transmitter channel units may beincorporated in the MAN 2.

In the receiver channel unit MR a signal is received by an antenna (notshown) and down-converted by a down-converter 42 which receives adown-conversion frequency signal from a receive frequency signalsynthesiser 44, the frequency of which is controlled by an MAN hardwareadaptation layer 46. The down-converted signal is demodulated by a 16QAMdemodulator 48 which outputs the parallel bit values of each symbol to aUW detection stage 50, where the timing of the received signal isdetected by identifying a unique word (UW) in the received signal. Thetiming information is sent to a frame and symbol timing unit 52 whichstores timing information and controls the timing of the later stages ofprocessing of the signal, as shown in FIG. 4. Once the block boundariesof the received data have been detected, the received blocks are turbodecoded by a decoder 54, descrambled by a descrambler 56 and output asreceived bursts to the HAL 46.

In the transmitter channel unit MT, data for bursts of 5 or 20 msduration are output from the HAL 46. Separate paths identified by thesuffixes a and b are shown in FIG. 4 for the 5 and 20 ms burstsrespectively. The data is scrambled by a scrambler 48 a/48 b and encodedby a turbo encoder 50 a/50 b. Unique Words (UW) are added as dictated bythe burst format at step 52 a/52 b and the resultant data stream ismapped onto the transmission signal set at step 54 a/54 b and filteredat step 56 a/56 b. The transmission timing is controlled at atransmission timing control step 58 a/58 b. At this step, the TDMA slotposition is controlled by a slot control step 60 according to adesignated slot position indicated by the HAL 46. A timing offset isoutput by the HAL 46 and is supplied to a timing adjustment step 62which adjusts the timing of the slot control step 60. This timing offsetis used to compensate for variations in propagation delay caused by therelative position of the MAN 2, the satellite 4 and the SAN 6 and iscontrolled by a signalling protocol, as will be described in greaterdetail below. The sets of data bits are output at a time determinedaccording to the slot timing and the timing adjustment to a 16QAMmodulator 64. The modulated symbols are upconverted by an upconverter 66to a transmission channel frequency determined by a frequency output bya transmission frequency synthesiser 68 controlled by the HAL 46. Theunconverted signal is transmitted to the satellite 4 by an antenna (notshown).

LESP Channel Format

FIG. 5 a shows the frame structure of one of the LESP channels. Eachframe LPF has a duration of 80 ms and has a header consisting of aconstant unique word UW which is the same for all frames. The uniqueword UW is used for frame acquisition, to resolve phase ambiguity of theoutput of the demodulator 48 and to synchronise the descrambler 56 andthe decoder 54.

FIG. 5 b shows the structure of each frame, which consists of the uniqueword UW of 40 symbols, followed by 88 blocks of 29 symbols each followedby a single pilot symbol PS, terminating in 8 symbols so as to make upthe total frame length to 2688 symbols, of which 2560 are data symbols.These data symbols are divided, as shown in FIG. 5 c, into two subframesSF1, SF2 each encoded separately by the encoder 16, each of 5120 bits,making 1280 symbols. The encoder 16 has a coding rate of 0.509375, sothat each subframe is encoded from an input block IB1, IB2 of 2608 bits,as shown in FIG. 5 d. This structure is summarised below in Table 1:

TABLE 1 LESP Frame Format Modulation 16QAM Data Rate (kbit/s) 65.2Interface frame length (ms) 80 Interface Frame Size (bits) 5120 Subframelength (ms) 40 Input Bits per Subframe 2608 Coding Rate 0.509375 OutputBit per Subframe 5120 Output Symbol Per Subframe 1280 Frame Length (ms)80 Data Symbol per Frame 2560 Pilot Symbol Insertion Rate 1/(29 + 1)Pilot Symbols per Frame 88 UW symbols 40 Frame Size 2688 Symbol Rate(ksym/s) 33.6MESP Channel Format

The MESP channel structure is based on 40 ms blocks with a channeltiming referenced to the timing of the associated LESP channel asreceived by the MAN's 2. Each 40 ms block can be divided into two 20 msslots, each of which can be further divided into four 5 ms slots, andthe division of each block into slots is determined flexibly by higherlevel protocols. FIG. 6 a shows the format of a 5 ms burst, consistingof a pre-burst guard time G1 of 6 symbols, a preamble CW of 4 symbols,an initial unique word UW1 of 20 symbols, a data subframe of 112symbols, a final unique word UW2 of 20 symbols and a post-burst guardtime G2 of 6 symbols.

The preamble CW is not intended for synchronisation purposes byreceivers (for example, the demodulators 30 a, 30 b) but convenientlyprovides a constant power level signal to assist the automatic levelcontrol of a high-power amplifier (HPA, not shown) in the transmittingMAN 2. In one example, each of the symbols of the preamble CW has thevalue (0,1,0,0). In an alternative format, the preamble may consist ofless than 4 symbols and the symbol times not used by the preamble CW areadded to the pre-burst and post-burst guard times G1, G2. For example,the preamble CW may be omitted altogether and the pre-and post-burstguard times increased to 8 symbols each.

The unique words include only the symbols (1,1,1,1), which is mappedonto a phase of 45° at maximum amplitude, and (0,1,0,1), which is mappedonto a phase of 225° at maximum amplitude. Hence, the unique words areeffectively BPSK modulated, although the symbols are modulated by the16QAM modulator 64. Indicating the (1,1,1,1) symbol as (1) and the(0,1,0,1) symbol as (0), the initial unique word UW1 comprises thesequence 10101110011111100100, while the final unique word UW2 comprisesthe sequence of symbols 1011101101011000011.

The 5 ms burst is designed for carrying short signalling messages ordata messages; the structure is summarised below in Table 2:

TABLE 2 5 ms Burst Structure Modulation 16QAM Input Bits per Burst 192Coding rate 3/7 Output Bits per Burst 448 Output Symbols per Subframe112 Preamble 4 Initial UW (symbols) 20 Final UW (symbols) 20 Totalsymbols 152 Total Guard Time (symbols) 12 Symbol Rate (ksym/s) 33.6 SlotLength (ms) 5

FIG. 6 b shows the structure of a 20 ms burst of the MESP channel. Thesame reference numerals will be used to denote the parts of thestructure corresponding to those of the 5 ms burst. The structureconsists of a pre-burst guard time G1 of 6 symbols, a preamble CW of 4symbols, an initial unique word UW1 of 40 symbols, a data subframe of596 symbols, a final unique word of 20 symbols and a post-burst guardtime G2 of 6 symbols. The structure is summarised below in Table 3:

TABLE 3 20 ms Burst Structure Modulation 16QAM Input Bits per Burst 1192Coding rate 1/2 Output Bits per Burst 2384 Output Symbols per Subframe596 Preamble 4 Initial UW (symbols) 40 Final UW (symbols) 20 Totalsymbols 660 Total Guard Time (symbols) 12 Symbol Rate (ksym/s) 33.6 SlotLength (ms) 20

The preamble CW has the same form and purpose as that of the 5 ms burst.The initial unique word UW1 comprises the sequence:

-   -   0000010011010100111000010001111100101101        while the final unique word UW2 comprises the sequence        11101110000011010010, using the same convention as that of the 5        ms burst.        MESP Timing Correction

As shown above, the MESP slot structure incorporates a very short guardtime of about 0.24 ms at each end. However, the difference in the SAN 6to MAN 2 propagation delay between the MAN 2 being at the sub-satellitepoint and at the edge of coverage is about 40 ms for a geostationarysatellite, so the position of each MAN 2 will affect the timing ofreception of transmitted bursts in the MESP channel, and may causeinterference between bursts from MAN's 2 at different distances from thesub-satellite point Moreover the satellite, although nominallygeostationary, is subject to perturbations which introduce a smallinclination to the orbit and cause the distance between the satellite 4and the SAN 6, and between the satellite 4 and the MAN 2, to oscillate.Although the position of the SAN 6 is fixed and that of the satellite 4can be predicted, the MAN's are mobile and therefore their positionschange unpredictably, and their clocks are subject to jitter and drift.

A timing correction protocol is used by the SAN 6 to measure thepropagation delay from the MAN 2 and send a timing correction value tothe MAN 2 to compensate for differences in propagation delay between thedifferent MAN's 2, so as to avoid interference between bursts fromdifferent MAN's caused by misalignment with the slots. The protocol willnow be illustrated with reference to the timing diagram of FIG. 7.

FIG. 7 shows LESP frames LPF including subframes SF1, SF2 and initialunique words UW. When the MAN 2 is switched on, or is able to acquireone of the LESP channels after an interval of not being able to do so,the MAN 2 receives (step 70) a 40 ms LESP subframe SF including returnschedule information which dictates the slot usage of a correspondingMESP channel. Return schedule information is transmitted periodicallywith a periodicity controlled by the SAN 6. The subframe SF includes thedesignation of a block of at least nine contiguous 5 ms slots as atiming acquisition group consisting of random access slots not assignedto any specific MAN 2. The MESP return schedule to which the subframe SFrelates begins 120 ms after the beginning of reception of the subframeSF. This 120 ms period allows 90 ms for the MAN 2 to demodulate the LESPsubframe SF (step 72) and 30 ms for the MAN 2 to initialise itself fortransmission (step 74).

At the beginning of the MESP return schedule there is allocated a timingallocation group of 5 ms slots. Initially, it is assumed that the MAN 2has the maximum timing uncertainty of 40 ms, corresponding to eight 5 msslots. Therefore, the MAN 2 can only transmit after the first eightslots of the timing acquisition group, and cannot transmit at all inacquisition groups containing less than nine slots, so as to avoidinterfering with transmissions in slots preceding the timing acquisitiongroup.

The MAN 2 randomly selects (step 78) one of the slots of the timingacquisition group following the first eight slots and transmits (step79) a burst in the selected slot, the burst including an indication ofthe slot selected. In the example shown in FIG. 7, the slots of thetiming acquisition group are numbered from 0 to M-1, where M is thenumber of slots in the timing acquisition group, and the number R,selected at random from 8 to M-1, is transmitted in the burst at step79. The burst may also indicate the type of the mobile, such asland-based, maritime or aeronautical.

The SAN 6 receives and records the time of arrival of the bursttransmitted by the MAN 2. From the slot number R indicated in the burst,the SAN 6 calculates the differential propagation delay to that MAN 2.Since the timing of transmission of the burst was (120+R×5) ms after thetime of reception of the LESP subframe SF, the timing of reception T_(R)of the burst is approximately (2×DP+C+120+5×R) ms after the time oftransmission of the LESP subframe LPSF, where DP is the differentialpropagation delay to that MAN 2 and C is a delay which is the same forall the MAN's in a group, and includes various factors such as thepropagation delay to and from the satellite 4 and the retransmissiondelay of the satellite 4. Hence, in this example, the differentialpropagation delay is calculated as:DP=T _(R) −C−120−5×R   (1)

The SAN 6 then transmits to the MAN 2 a data packet indicating a timingcorrection offset X in the range 0 to 40 ms. The offset replaces theinitial timing offset of 40 ms in step 76, for subsequent transmissions.The MAN 2 receives the timing correction offset and adjusts itstransmission timing accordingly.

If the burst transmitted by the MAN 2 interferes with a bursttransmitted by another MAN 2 also attempting to receive a timingcorrection, the SAN 6 may not be able to read the contents of eitherburst and in that case will not transmit a timing offset correction toeither MAN 2. If the MAN 2 does not receive a timing offset correctionfrom the SAN 6 within a predetermined time, the MAN 2 waits for a randominterval within a predetermined range before attempting to transmit aburst in the next subsequently available timing acquisition group. Thepredetermined range of intervals is determined by a signalling packettransmitted by the SAN 6 which indicates maximum and minimum intervalsto be observed by MAN's 2 after a first unsuccessful transmission beforeattempting transmission, together with a further waiting interval to beadded to the total waiting interval each time a further retransmissionis made following an unsuccessful transmission.

FIG. 8 a illustrates the transmission timing of one of the MAN's 2 whichhas previously received a timing correction offset value X. As in FIG.7, the MAN 2 receives (step 80) the LESP subframe SF which includesreturn schedule information. The MAN 2 demodulates (step 82) the LISPsubframe LPSF and initialises (step 84) its transmitting channel unit,during a total allotted time of 120 ms after the beginning of receptionof the LESP subframe LPSF. The MAN 2 calculates the start of the MESPreturn schedule as being (120|X) ms from the beginning of reception ofthe subframe SF which carries the return schedule information. The MAN 2therefore waits for the timing offset period X (step 86) after the endof the 120 ms period before being able to transmit.

In this example, the return schedule dictated by the LESP subframe LPSFincludes a four 5 ms slots, followed by a 20 ms slot. If the MAN 2 hasbeen allocated a 20 ms slot, then it will transmit (step 88) in thedesignated 20 ms slot; if the MAN 2 has been allocated a 5 ms slot, thenit will transmit in the designated 5 ms slot. Alternatively, if the 5 msslots are designated as being random access slots and the MAN 2 has ashort packet that is due to be sent to the SAN 6, the MAN 2 selects oneof the four slots at random and transmits in that slot (step 89).

If the SAN 6 detects from the transmission by the MAN 2 that acorrection in the timing offset is needed, for example if the timebetween the start of the burst and the slot boundary as measured by theSAN 6 is less than a predetermined number of symbols, the SAN 6indicates a new timing correction to the MAN 2 in a subsequent datapacket. This may be indicated as an absolute timing offset X or as arelative timing offset to be added or subtracted from the current valueof X.

Timing Uncertainty

In the timing correction offset burst the SAN 6 transmits to the MAN 2,together with the timing offset, a timing uncertainty rate R_(U)indicating the rate at which the timing of the MAN 2 is likely tochange. For example, the timing uncertainty rate may represent a numberof symbols pet second by which the MAN 2 is likely to change its timing.The SAN 6 determines the timing uncertainty rate from the class of theMAN 2 (e.g. land mobile, aeronautical) and other factors such as theinclination of the orbit of the satellite 6.

The MAN 2 times the interval elapsed since the last timing correctionwas received and multiplies this by the timing uncertainty rate R_(U) togive a timing uncertainty t_(U), wheret _(U)=MIN(T−T _(C) ×R _(U), 40 ms)   (2)where T is the current time and T_(C) is the time at which the lastcorrection was received. The MIN function means that the timinguncertainty cannot exceed the maximum uncertainty of 40 ms.

The timing offset X is reduced by the timing uncertainty t_(U) suchthat:X=MIN(X _(C) −t _(U), 0)   (3)where X_(C) is the initial value of X indicated in the last timingcorrection, the MIN function ensuring that X cannot fall below zero.

FIG. 8 b illustrates the transmission timing of one of the MAN's 2 withtiming uncertainty. Steps 80 to 84 correspond to those shown in FIG. 8 aand their description will not be repeated. At step 86, the MAN 2calculates the MESP return schedule as starting (120+X) ms after thebeginning of reception of the subframe SF, using the value of X asreduced by the timing uncertainty t_(U). As a result of the timinguncertainty t_(U), the MAN 2 must ignore the first I slots of a randomaccess group, whereI=INT[(t _(S) −t _(G) +t _(U))/t _(S)]  (4)t_(S) is the slot duration of 5 ms and t_(G) is the guard time G1, whichis 6 symbol periods in this case.

In the example shown in FIG. 8 b, there are four 5 ms slots at the startof the MESP return schedule, but t_(U) is 7 ms, so that the first twoslots must be ignored. The MAN 2 can then only transmit in the third andfourth slots.

If the timing uncertainty t_(U) is greater than a predetermined value,such as the value of the guard time, the MAN 2 reverts to the randomaccess timing correction request process shown in FIG. 7 and inhibitstransmission in time slots allocated exclusively to itself, except wherea sufficient number of these are concatenated so that their total lengthcan accommodate both the timing uncertainty and the burst itself, untila new timing correction offset has been received from the SAN 6.However, the protocol differs from that of FIG. 7 in that the MAN 2 usesits current timing offset X instead of returning to the default value of40 ms in step 76. This protocol reduces the chance of interferencebetween bursts in allocated slots.

In the above embodiment, the timing offset X is reduced by the timinguncertainty t_(U) for all transmissions by the MAN 2. In an alternativeembodiment, the timing offset X is reduced by the timing uncertaintyt_(U) only for transmissions by the MAN 2 in random access slots, whilethe original timing offset X_(C) received in the last timing correctionmessage from the SAN 6 is applied when transmitting in allocated slots.In this alternative embodiment, it is important to distinguish betweentiring correction messages initiated by the SAN 6, after detection of atransmission by the MAN 2 in an allocated slot too close to the slotboundary, and timing correction messages sent by the SAN 6 in responseto a timing correction request by the MAN 2, which will have a differenttiming offset from the transmissions in allocated slots. Therefore, theSAN 6 indicates in the timing correction message whether this is beingsent in response to a request by the MAN 2, or was initiated by the SAN6. The MAN 2 then determines the new timing offset X_(C) from the timingoffset indicated in the timing correction message according to how thetiming correction message was initiated.

Unique Word Structure

As shown in FIGS. 6 a and 6 b, each MESP burst includes an initialunique word UW1 and a final unique word UW2. This format is particularlyadvantageous for TDMA channels with short guard times between slots. Byway of comparison, FIGS. 9 a to 9 c show a conventional burst structurewith initial unique word only, respectively with no collision, two-burstcollision and three-burst collision, while FIGS. 10 a to 10 c show theequivalent situations with both an initial and final UW structure.

As shown in FIG. 9 b, if burst 2 transmitted in slot 2 is delayedbecause of timing error, the data contents of burst 2 interfere with theUW of burst 3 in slot 3 and may both be corrupted, possibly preventingthe data contents of burst 3 from being read correctly as a result of afailure to acquire the symbol timing of burst 3. However, in thesituation shown in FIG. 10 b, the final UW of burst 2 interferes withthe initial UW of burst 3, but in both bursts the data and one of theunique words is uncorrupted, giving a good chance of reading both databursts.

In the situation shown in FIG. 9 c, burst 2 is delayed and burst 4 isadvanced, both as a result of timing errors. The data content of bursts2 and 3, and the unique words of bursts 3 and 4, are corrupted so thatit will be difficult to read any of the data contents of bursts 2 to 4.In contrast, in the situation shown in FIG. 10 c, the final UW of burst2, both unique words of burst 3 and the initial unique word of burst 4are corrupted. Nevertheless, if the timing of bursts 2 and 4 can beacquired from the uncorrupted unique words, the corrupted unique wordsof bursts 2 and 4 can be synthesised and subtracted from the receivedsignal of burst 3, allowing the corrupted unique words of burst 3 to berecovered and the data content of burst 3 to be read successfully.

The use of two unique words per burst also provides the advantages oftime diversity: in the presence of fading or impulsive noise, the chanceof two separate unique words being corrupted is less than that of oneunique word of the combined length. The two unique words can be detectedindependently and the results combined before a timing decision is made.

In order to demodulate a received burst, the SAN 6 needs to estimate thecarrier amplitude, phase and frequency. The estimated channel state isalso used by the decoder 38 a/38 b. Since there is an UW present at boththe beginning and end of each burst, the channel state at both thebeginning and end of the burst can be determined, and optionally thechannel state throughout the burst can be interpolated from these. Thismay result in improved demodulation and decoding performance.Furthermore, timing slip between the beginning and the end of the databurst can be detected; this is advantageous where there is considerabledrift in the transmitter or receiver clock Commonly, the channel statecannot be estimated from the data burst itself, because the energy ofthe data portion is typically too low.

As a further advantage, the proposed unique word structure givesimproved performance with high-power amplifiers (HPA). A common problemwith HPA's is their slow ramp-up/down at the beginning and end of aburst. This may result in distortion or attenuation of symbols at thestart and end of a burst. If these symbols were carrying encoded data,their distortion could lead to loss of the whole encoded data in theburst. With the proposed structure, only some of the UW symbols will bedistorted, which is less likely to cause loss of the whole burst.

As a less advantageous alternative, additional fields may be transmittedin each burst either before the initial UW or after the final UW of theburst, or both. The additional fields may be additional data fieldscarrying additional data or signalling, or may carry further burstformat signals designed to assist in the demodulation and/or decoding ofthe data content of the burst. However, such additional fields arevulnerable to interference and preferably should not carry data orsignalling essential for demodulation and/or decoding of the burst.

The above embodiments have been described with reference to certainInmarsat™ systems purely by way of example and aspects of the presentinvention are in no way limited thereto. For example, it will be readilyunderstood to the skilled person that the problem of timing correctionoccurs in geo-stationary, geosynchronous and non-geostationary satellitesystems and aspects of the present invention are applicable to thesesystems. Moreover, timing errors can occur for reasons such as clockinstability as well as relative movement between satellites, basestations and wireless transceivers, so that aspects of the presentinvention are also applicable to wireless communication systems notusing satellites as relay stations, such as terrestrial communicationssystems or systems involving alternative relay stations such as balloonsor other aircraft.

Although the above embodiments have been described with reference to aTDMA channel format, it will be readily understood by the skilled personthat the problem of interference as a result of timing error can occurwith other channel formats, such as combined TDMA-CDMA, slotted Alohaand other time-divided formats and that aspects of the present inventionare also applicable to such formats.

The description of the above embodiments includes a detailed descriptionof the transmission formats of LESP and MESP channels. Aspects of thesechannel formats are particularly advantageous for packet datatransmission via satellite, particularly via geostationary satellite andhave been selected after considerable investigation of alternativeformats, but may also be advantageous in different contexts. On theother hand, it will be apparent that some aspects of the presentinvention are entirely independent of the specific channel formats used.

While the apparatus of the specific embodiments has been described interms of functional blocks, these blocks do not necessarily correspondto discrete hardware or software objects. As is well known, mostbaseband functions may in practice be performed by suitably programmedDSP's or general purpose processors and the software may be optimisedfor speed rather than structure.

1. A method of controlling the transmission of data over a time-dividedmultiple access channel of a wireless communications link, comprising:determining an allocation scheme of said channel to each of a pluralityof transceivers, and transmitting said allocation scheme to saidtransceivers, wherein said transceivers transmit data in said channelwith a format including periodic blocks of constant length each occupiedby either one long burst or a plurality of short bursts of equal lengthand wherein a length for each long burst occupying a block is constant,whereby the division of each block into either one long burst or aplurality of short bursts is determined flexibly.
 2. The method of claim1, further comprising: transmitting the data in one or more short burstsand/or one or more long bursts, the short bursts comprising 112modulated data symbols and having a total length of approximately 5 ms,and the long bursts comprising 596 data symbols and having a totallength of approximately 20 ms, and whereby the division of each blockinto either one long burst or a number of short bursts is determinedflexibly.
 3. A first transceiver that receives an allocation scheme fora time-divided multiple access channel to each of a plurality oftransceivers including the first transceiver, wherein the allocation isreceived by each of the plurality of transceivers, and that transmits awireless link signal having a format including periodic blocks ofconstant length each occupied by either one long burst or a plurality ofshort bursts of equal length and wherein a length for each long burstoccupying a block is constant, whereby the division of each block intoeither one long burst or a plurality of short bursts is determinedflexibly.